This invention relates generally to the field of thermal barrier coatings used to insulate substrate materials from high temperature environments, such as ceramic coatings applied to metal substrates. This invention has specific application for a ceramic thermal barrier coating applied to a superalloy component of a gas turbine engine.
It is well known to apply a thermal barrier coating (TBC) to a substrate material to inhibit the flow of heat into the substrate. Such coatings commonly protect alloy components of gas turbine engines that are exposed to the hot combustion gas.
Ceramic thermal barrier coating materials may be applied to a metal alloy substrate by a vapor deposition process, such as electron beam physical vapor deposition (EB-PVD). A ceramic layer deposited by vapor deposition may form a columnar-grained structure, wherein a plurality of individual columns of directionally solidified ceramic material are separated by small gaps extending through essentially the entire thickness of the TBC layer. Individual columns may be about 10-30 microns wide and the gaps between columns may be about 1-2 microns wide. One such approach is described in U.S. Pat. No. 4,405,659 to Strangman. The gaps between the various columns of material function to relieve stress in the material, thereby reducing its susceptibility to failure caused by thermal shock. Unfortunately, EB-PVD is known to be an expensive process. Furthermore, the gaps between the columns provide pathways for penetration of contaminants from a high temperature environment, thereby reducing the effectiveness of the insulating layer and facilitating the oxidation or corrosion of the underlying bond coat and/or substrate material.
It is known to apply a ceramic thermal barrier coating material by an air plasma spray (APS) process. Such coatings are formed by heating a gas-propelled spray of a powdered metal oxide or non-oxide material with a plasma spray torch. The spray is heated to a temperature at which the powder particles become molten. The spray of molten particles is directed against a substrate surface where they solidify upon impact to create the coating. The conventional as-deposited APS microstructure is known to be characterized by a plurality of overlapping splats of material, wherein the inter-splat boundaries may be tightly joined or may be separated by gaps resulting in some porosity. The individual splats of the conventional as-deposited APS microstructure are characterized by intra-splat columns of directionally solidified material extending through the thickness of the splat, typically 2-5 microns. This structure is referred to hereinafter as xe2x80x9cconventional as-deposited APS microstructure.xe2x80x9d Such coatings are generally less expensive to apply than EB-PVD coatings and they provide a better thermal and chemical seal against the surrounding environment than do columnar-grained structures. However, unlike the columnar-grained structure, the inter-splat gaps in the conventional as-deposited APS microstructure tend to densify upon exposure to high temperatures and fast temperature transients. Such densification may result in a shorter operating life in a gas turbine environment.
It is possible to achieve a columnar grained structure by using an APS process to deposit a ceramic thermal barrier coating, as described in U.S. Pat. No. 5,830,586 to Gray, et al., incorporated by reference herein. These gaps may be 200-300 microns apart with fully dense columnar material there between. Although the spacing of these gaps is somewhat different than the gaps in the columnar grained structure obtained by EB-PVD, such gaps still provide strain tolerance for the materials. Accordingly, the term xe2x80x9ccolumnar grained materialxe2x80x9d as used herein is meant to encompass all such structures regardless of the method of deposition. Gray teaches that when the temperature of the particle-receiving surface is controlled to a desired high temperature, the overlapping layers of deposited material will flow together in a micro-welding process to form a columnar ordering of the adjacent particle layers. While such a structure may have some advantages when compared to the conventional as-deposited APS microstructure, the high temperature necessary for deposition can cause oxidation of the underlying bond coat during the deposition process, thereby resulting in poor bonding between the thermal barrier coating layer and the bond coat and early failure of the TBC.
It is also known to use a quenching process to create a fine network of cracks in a plasma flame sprayed ceramic thermal barrier coating, as described in U.S. Pat. No. 4,457,948 to Ruckle. The network of cracks serve to relieve strain in the material, thereby improving its performance under thermal transient conditions. While the plasma spray processes described by Gray and Ruckle may be less expensive than an EB-PVD process, the resulting coatings still suffer from the disadvantages described above due to the encroachment of the high temperature environment through the strain-relieving gaps or cracks.
U.S. Pat. No. 5,576,069 to Chen, et al., describes a laser re-melting process for improved sealing of a plasma-sprayed thermal barrier coating. A high power laser beam is used to melt a thin layer on a surface of a plasma-sprayed coating. The glazed surface provides an improved seal against an oxidizing environment. However, such a coating lacks the thermal stress compliance of a columnar-grained coating.
The present invention includes a strain-tolerant thermal barrier coating and method of making such a coating for protecting an article from exposure to a high temperature oxidizing environment.
A thermal barrier coating is described herein as including: a first layer of ceramic insulating material having a conventional as-deposited APS microstructure; and a second layer of ceramic insulating material disposed on the first layer, the second layer having a columnar-grained structure. The first layer may have a density of no more than 70-85% of its theoretical density. The second layer may have a density of at least 85% of its theoretical density. A sinter-inhibiting material may be disposed on the second layer between adjacent columns of the columnar-grained structure.
An article having a thermal barrier coating is described herein as including: a substrate having a surface; a thermal barrier coating deposited over the surface of the substrate, the thermal barrier coating further comprising: a first layer of ceramic insulating material having a conventional as-deposited APS microstructure disposed over the substrate surface; and a second layer of ceramic insulating material disposed on the first layer, the second layer having a columnar grained structure. A bond coat material may be deposited between the substrate surface and the first layer of ceramic insulating material.
A method of insulating a substrate is described herein as including: depositing a first layer of a ceramic insulating material over the substrate using an air plasma spray process to obtain a conventional as-deposited APS microstructure in the first layer; and depositing a second layer of a ceramic insulating material over the first layer using a process that results in a columnar-grained structure in the second layer. The first layer may be deposited to have a density of no more than 70-85% of its theoretical density. The second layer may be deposited to have a density of at least 85% of its theoretical density. The method may further include depositing a sinter-inhibiting material on the second layer between adjacent columns of the columnar-grained structure. The first layer may be deposited to have pores so that the pores in the first layer arrest the propagation of cracks originating in the second layer.